1. Field of the Invention
The present invention relates to a toothed belt, which may be a so-called timing belt or synchronous belt, used as a power transmission belt in, for example, a prime mover of a vehicle such as an automobile, to transmit a rotational force of a crank shaft thereof to a cam shaft, a balancer shaft, a drive shaft of a fuel injection pump and so on.
2. Description of the Related Art
Conventionally, such a toothed belt comprises a toothed rubber layer having teeth formed on a surface thereof, a back rubber layer adhered to the other surface of the toothed rubber layer, and a plurality of cord elements embedded in a boundary between the toothed rubber layer and the back rubber layer.
Recently, with the advance in performance of a prime mover of a vehicle, such as an automobile, the number of revolutions of the prime mover has increased. The toothed belt used to rotationally drive a cam shaft, a fuel injection pump or the like is subjected to a larger load.
Breakage of the teeth of the toothed belt is prematurely caused because of the larger stress and strains of an increasing load to which the toothed belt is subjected and, consequently, the service life of the toothed belt is further shortened.
Therefore, an object of the present invention is to provide a toothed belt which is constituted such that a premature breakage of teeth of the toothed belt can be prevented, to prolong the service life of the toothed belt.
In accordance with the present invention, a toothed belt comprises: a toothed rubber layer having teeth formed on a surface thereof and containing a plurality of short fibers entirely distributed therein; a cover fabric covering the surface of said toothed rubber layer on which the teeth are formed; a back rubber layer integrally applied to another surface of said toothed rubber layer; and a plurality of cord elements disposed across a width of the belt and intervened between said toothed rubber layer and said back layer in such a manner that a portion of each cord element is embedded in said toothed rubber layer, and the remaining portion thereof is embedded in said back rubber layer. The short fibers are entirely distributed in said toothed rubber layer, and are regularly oriented.
The short fibers may be substantially oriented in a length direction in said toothed rubber layer. In other words, the short fibers may be distributed in the length direction in said toothed rubber.
The short fibers may be oriented along a toothed-profile surface of the teeth of said toothed rubber layer, at a peripheral area of said toothed rubber layer, adjacent to the toothed-profile surface thereof. In this case, the short fiber, at a central area of said teeth of said toothed rubber layer, may be substantially oriented perpendicularly to a surface of said toothed rubber layer, to which said back rubber layer is applied.
On the other hand, the short fibers, in said toothed rubber layer, may be substantially oriented in a width direction thereof.
The short fiber may be comprised of short aramid fibers. In this case, the short aramid fibers may be either short meta-aramid fibers or short para-aramid fibers. The fibers may have a length of approximately 1 to approximately 6 mm. Preferably, the fibers have a length of approximately 3 mm.
An amount of said short fibers is in a range of approximately 3 to approximately 30 phr with respect to 100 phr of a rubber stock for said toothed rubber layer. Preferably, an amount of said short fibers is approximately 6.5 phr with respect to 100 phr of a rubber stock for said toothed rubber layer.
A rubber stock for said toothed rubber layer and said back rubber layer may be comprised of hydrogenated nitrile rubber having a hydrogenation rate of no less than 91%. The rubber stock may contain an organic peroxide-type vulcanizing agent. Also, the rubber stock may contain sulfur as a vulcanizing agent, and a sulfur-type vulcanizing accelerator.
Preferably, the cover fabric and a rubber stock for said toothed rubber layer are preformed into a profile analogous to a finished profile of the toothed belt.
The cover fabric may be comprised of a weave of stretchable composite yarns, laid along a length direction of the toothed belt, and nonstretchable yarns, laid along a width direction of the toothed belt. In this case, said cover fabric should exhibit an elasticity so that said cover fabric is only broken when stretching said cover fabric from between approximately 30% to approximately 80% with respect to an original size thereof. The cover fabric may be treated with a resonocinol formaldehyde latex solution.
Each of said cord elements may be formed of high strength glass fibers, and these cord elements may be arranged at a cord gap of approximately 0.17 to approximately 0.28 mm.
Also, each of the cord elements may be formed of aramid, and these cord elements may be arranged at a cord gap of approximately 0.25 to approximately 0.36 mm.
Preferably, the cord elements are treated with a resorcinol formaldehyde latex solution, a rubber solution and a solution of cashew-modified phenol resin.